This invention relates to a method for simultaneously chemically etching an array of uniform through holes in a substrate and more particularly, the invention relates to the fabrication of nozzles for ink jet printers.
In prior art ink jet printing systems, it is known that an array of closely spaced nozzles is required. During ink jet printing with this system, drops are simultaneously ejected by all the nozzles and a charge electrode is arranged in front of each nozzle in the area where the drops are formed. A constant deflecting field is operable to deflect all the charged drops from the ink stream so that only the uncharged drops continue to the paper to form printed characters. An ink jet printer with an array of several nozzles is described in U.S. Pat. No. 3,373,437.
It has also been known in the prior art that a nozzle array suitable for use in ink jet printing can be fabricated by etching a semiconductor chip. One example of such an ink jet nozzle is described in U.S. Pat. No. 4,007,464 to Bassous et al, which patent is assigned to the assignee of the present invention. In this process one surface of the semiconductor chip is initially photoresist coated and then exposed and developed so that the etchant attacks only predetermined regions to produce the nozzles. Those nozzles have a pyramidally tapered cross-section.
Unfortunately, it is very difficult to produce substrates having a totally uniform thickness from semiconductor materials. Non-uniform thicknesses are particularly detrimental when several tapered nozzles are to be etched simultaneously. In this cse, a thin area of the substrate will have a larger orifice than in a thicker section, if both nozzles are etched simultaneously. Because of their improved characteristics, tapered such as conical or pyramidal nozzles such as described in the above-mentioned Bassous et al patent, are desirable for ink jet printers.
IBM technical disclosure bulletin Vol. 17 No. 11 April 1975 pages 3450-52 describes a method for simultaneously etching several nozzles in a silicon plate which produces uniform orifice widths irrespective of differences in the thickness of the silicon plate. Differences in the thickness of the plate are compensated for during the exposure of the photoresist layer by changing the size of the exposure area or, in the case of square exposure surfaces, by tilting them in relation to the direction of the silicon plate. These known methods of changing the surface attacked by the etchant necessitate that the photoresist free area associated with each nozzle be produced separately and that the thickness of the silicon plate be measured in those areas where a nozzle is to be etched. These factors make this method unsuitable for use in a production fabrication technique for ink jet nozzles.
Another method for compensating for the non-uniform thickness of the semiconductor substrate is described in application Ser. No. 781,616, filed Mar. 28, 1977, now U.S. Pat. No. 4,066,491, and assigned to the assignee of the present invention. The holes are first chemically pre-etched until the first hole is about to penetrate through the substrate and subsequently sputter etching is utilized until all holes have penetrated through the substrate. This method requires two separate etching steps which adds to the complexity of the method.
A further method for solving the problem of non-uniform semiconductor substrates in the production of ink jet nozzles is described in Application Ser. No. 805,675 filed on June 13, 1977, now abandoned, and assigned to the assignee of the present invention. In this application, the substrate is anisotropically etched from one side of the wafer until the openings are through in the thin section of the wafer. Apertures are then made in the masking material on the other side of the wafer in the locations where the nozzles are desired. The substrate is then anisotropically etched again so that the array of nozzles is then formed to the size of the apertures opened on the other side of the substrate. This method also requires two etching steps and for this reason has not proved entirely satisfactory in making nozzle arrays for ink jet printers.